The invention relates to means for the use of an anticoagulant in connection with autotransfusion of blood. Customarily, an intraoperative autotransfusion system includes a suction wand for aspirating blood from an open wound site, a blood reservoir for collecting the aspirated blood, and a flexible conduit for conducting the blood from the wand to the reservoir. A known method of autotransfusion includes meeans for subjecting the interior of the blood reservoir to a source of vacuum effective through the flexible conduit for inducing blood to be aspirated into the wand and to flow from the wand, through the conduit into the reservoir. During handling of blood in an extracorporeal device such as an autotransfusion system, the blood readily tends to clot, and block the various structures through which it must flow, and thus may not be acceptable for reinfusion into the patient. Thus, it is often desirable to add an effective anticoagulant to the blood as, or promptly after, it is aspirated so that a mixture of blood and anticoagulant flows through the apparatus and may then be infused into the patient. The desirable proportion of anticoagulant to blood is generally fixed within a specific range. Anticoagulant can be added through a side port of the wand into a blood flow passageway or lumen where it mixes with the blood as it flows through the wand during aspiration. Commonly, a tube connects the site of anticoagulant entry at the wand with a source of anticoagulant positioned at a particular, generally fixed, height.
With such an anticoagulant delivery system, we have found that the delivery of anticoagulant to the blood flowing through the wand depends upon two key factors involving the height at which the wand is operated (i.e., the height of the wound site) relative: (1) to the height of the anticoagulant source; and (2) to the height of the blood inlet to the blood reservoir. This height sensitivity can be explained as follows. First, the driving force for anticoagulant delivery from the anticoagulant source to the blood flowing through the wand generally depends upon several factors, including the hydrostatic head of anticoagulant associated with the height difference between the anticoagulant source and the site of anticoagulant entry into the wand. Second, the flow rate of blood past the site of anticoagulant entry into the wand can affect the rate of anticoagulant delivery. The driving force for such blood flow from the wound site to the blood reservoir depends upon the sum of two pressures: the vacuum level within the blood reservoir; and the hydrostatic head of blood (within the line connecting the wand and the blood reservoir) associated with the height difference between the wand tip and the blood entry point into the blood reservoir. A variation in the hydrostatic pressure within the blood line can affect the flow rate of the blood, which in turn affects the driving force for anticoagulant flow into the wand.
Under a number of possible conditions, the height location of the wand at the aspiration site relative to both the blood reservoir and anticoagulant source can be variable. These conditions may include: inaccurate positioning during the initial set-up of the blood reservoir and/or anticoagulant source relative to the wound or blood aspiration site; and, after device set-up, a change in the wound site at which the wand is being used, or a repositioning of the patient. In any of these cases, the flow of anticoagulant into the wand for mixing with the blood in the above-described devices will tend to vary with the change in elevation of the wand relative to the anticoagulant source and blood reservoir, if they remain at fixed heights. This could alter the proportion of anticoagulant to the blood, and the desired range of said proportion may be exceeded.
The anticoagulant system described herein is particularly designed to compensate for the effects of changes in the aspirating wand elevation relative to the anticoagulant source and relative to the blood reservoir.